Lubricating Oil Composition

ABSTRACT

Disclosed is a lubricating oil composition comprising: a lubricating base oil comprising 0.5% to 70% by mass of an ester base oil based on the total amount of the lubricating base oil, and having a kinematic viscosity at 40° C. of 18 to 28 mm 2 /s; and an organic molybdenum compound in an amount of 100 to 1000 mass ppm in terms of the molybdenum element based on the total amount of the lubricating oil composition, wherein the lubricating oil composition has a kinematic viscosity at 40° C. of 50 mm 2 /s or less.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition.

BACKGROUND ART

In recent years, in order to cope with environmental problems such as reduction in carbon dioxide emission, the saving of energy in automobiles, construction machineries, agricultural machineries, etc., namely, fuel saving has become an urgent task, and thus, it has been strongly desired that devices such as engines, transmissions, final drive gears, compressors or hydraulic systems contribute to such saving of energy. As such, lubricating oil used in these devices has been required to reduce stirring resistance and rotational resistance, in comparison to the conventional oil.

As a means for fuel saving in transmissions and final drive gears, reduction in the viscosity of lubricating oil is applied. For example, among transmissions, automatic transmissions and continuously variable transmissions for use in automobiles have a torque converter, a wet clutch, a gear bearing mechanism, an oil pump, a hydraulic control mechanism and the like, and also, manual transmissions and final drive gears have a gear bearing mechanism, and by reducing the viscosity of lubricating oil used in these devices, the stirring resistance and rotational resistance of such a torque converter, a wet clutch, a gear bearing mechanism, an oil pump and the like can he reduced, and the power transmission efficiency can be improved, so that the improvement of fuel consumption in automobiles can be achieved.

However, in order to reduce the viscosity of lubricating oil and to achieve a high viscosity index, if the viscosity of base oil is decreased and a large amount of viscosity index improver is blended into the oil, a reduction in the oil film thickness, which is competing performance, causes a reduction in extreme pressure properties and wear resistance, and as a result, seizure and the like may occur, so that defects and the like would occur in transmissions, etc. Moreover, if the amounts of a sulfur extreme pressure agent and a phosphorus-sulfur extreme pressure agent are increased to improve extreme pressure properties, oxidation stability would be significantly deteriorated.

As conventional lubricating oil compositions, lubricating oil compositions produced by blending various types of additives to mineral oil- and/or synthetic oil-based lubricating base oil, which have both fuel saving and the sufficient durability of gears, bearings, etc. have been proposed (see, for example, Patent Literatures 1 and 2). However, regarding fuel saving, there has been still room for improvement even in these conventional lubricating oil compositions.

CITATION LIST Patent literature

Patent Literature 1: JP 2008-208212 A

Patent Literature 2: JP 2009-249496 A

SUMMARY OF INVENTION Technical Problem

The present invention has been made under the aforementioned circumstances, and it is an object of the present invention to provide a lubricating oil composition, which has extreme pressure properties and wear resistance enough to achieve fuel saving, and reduces a metal-to-metal friction coefficient.

Solution to Problem

To solve the aforementioned object, the present invention provides a lubricating oil composition according to [1] to [4] below, use of the composition according to [5] below, and use of the composition for the production according to [6] below.

[1] A lubricating oil composition comprising: a lubricating base oil comprising 0.5% to 70% by mass of an ester base oil based on the total amount of the lubricating base oil, and having a kinematic viscosity at 40° C. of 18 to 28 mm²/s; and an organic molybdenum compound in an amount of 100 to 1000 mass ppm in terms of molybdenum element based on the total amount of the lubricating oil composition, wherein the lubricating oil composition has a kinematic viscosity at 40° C. of 50 mm²/s or less.

[2] The lubricating oil composition according to [1] above further comprising 2% by mass or more of a copolymer consisting of an α-olefin and an ester monomer having a polymerizable unsaturated bond, wherein the weight-average molecular weight of the copolymer is 2000 to 20000.

[3] The lubricating oil composition according to [1] or [2] above further comprising a boron-containing dispersant in an amount of 100 to 500 mass ppm in terms of boron element based on the total amount of the lubricating oil composition.

[4] The lubricating oil composition according to any one of [1] to [3] above, which is used for a hypoid gear.

[5] Use of the composition as a lubricating oil for a hypoid gear, wherein the composition comprises: a lubricating base oil comprising 0.5% to 70% by mass of an ester base oil based on the total amount of the lubricating base oil, and having a kinematic viscosity at 40° C. of 18 to 28 mm²/s; and an organic molybdenum compound in an amount of 100 to 1000 mass ppm in terms of the molybdenum element based on the total amount of the lubricating oil composition, wherein the lubricating oil composition has a kinematic viscosity at 40° C. of 50 mm²/s or less.

[6] Use of the composition for the production of a lubricating oil for a hypoid gear, wherein the composition comprises: a lubricating base oil comprising 0.5% to 70% by mass of an ester base oil based on the total amount of the lubricating base oil, and having a kinematic viscosity at 40° C. of 18 to 28 mm²/s; and an organic molybdenum compound in an amount of 100 to 1000 mass ppm in terms of the molybdenum element based on the total amount of the lubricating oil composition, wherein the lubricating oil composition has a kinematic viscosity at 40° C. of 50 mm²/s or less.

The term “kinematic viscosity” used in the present invention means a kinematic viscosity defined according to ASTM D-445. In addition, the term “viscosity index” used in the present invention means a viscosity index measured in accordance with JIS K 2283-1993.

Advantageous Effects of Invention

According to the present invention, there is provided a lubricating oil composition, which has sufficient extreme pressure properties and wear resistance, and further reduces a metal-to-metal friction coefficient. Therefore, when the lubricating oil composition of the present invention is applied to an automotive manual transmission, an automatic transmission or a continuously variable transmission, or to an industrial gear system, it can achieve fuel saving, while maintaining properties necessary as a gear oil, and particularly, as a hypoid gear oil.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be described.

The lubricating oil composition according to the present embodiment comprises: (A) a lubricating base oil comprising 0.5% to 70% by mass of an ester base oil based on the total amount of the lubricating base oil, and having a kinematic viscosity at 40° C. of 18 to 28 mm²/s; and (B) an organic molybdenum compound in an amount of 100 to 1000 mass ppm in terms of the molybdenum element based on the total amount of the lubricating oil composition, wherein the lubricating oil composition has a kinematic viscosity at 40° C. of 50 mm²/s or less.

[Component (A): Lubricating Base Oil]

The lubricating oil composition of the present embodiment comprises (A) a lubricating base oil comprising 0.5% to 70% by mass of an ester base oil based on the total amount of the lubricating base oil, and having a kinematic viscosity at 40° C. of 18 to 28 mm²/s.

The alcohol constituting the ester base oil may be either monohydric alcohol or polyhydric alcohol (polyol), and the acid constituting the ester base oil may be either monobasic acid or polybasic acid. In addition, as long as the ester base oil is a base oil containing an ester bond, it may also be a complex ester compound.

As monohydric alcohols, the monohydric alcohol having generally 1 to 24, preferably 1 to 12, and more preferably 1 to 8 carbon atoms is used, and such alcohol may be linear or branched, and may also be saturated or unsaturated. Specific examples of the alcohol having 1 to 24 carbon atoms include methanol, ethanol, linear or branched propanol, linear or branched butanol, linear or branched pentanol, linear or branched hexanol, linear or branched heptanol, linear or branched octanol, linear or branched nonanol, linear or branched decanol, linear or branched undecanol, linear or branched dodecanol, linear or branched tridecanol, linear or branched tetradecanol, linear or branched pentadecanol, linear or branched hexadecanol, linear or branched heptadecanol, linear or branched octadecanol, linear or branched nonadecanol, linear or branched icosanol, linear or branched henicosanol, linear or branched tricosanol, linear or branched tetracosanol, and the mixtures thereof.

As polyhydric alcohols (polyol), generally a 2- to 10-valent, and preferably a 2- to 6-valent polyhydric alcohol is used. Specific examples of the 2- to 10-valent polyhydric alcohol include: divalent alcohols such as ethylene glycol, diethylene glycol, polyethylene glycol (timer to 15-mer of ethylene glycol), propylene glycol, dipropylene glycol, polypropylene glycol (trimer to 15-mer of propylene glycol), 1,3-propanediol, 1,2-propanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl -1,2-propanediol, 2-methyl-1,3-propanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, or neopentyl glycol; polyhydric alcohols such as glycerin, polyglycerin (dimer to octamer of glycerin, such as diglycerin, triglycerin, or tetraglycerin), trimethylolalkane (trimethylolethane, trimethylolpropane, trimethylolbutane, etc.) and the dimer to octamer thereof, pentaerythritol and the dimer to tetramer thereof, 1,2,4-butanetriol, 1,3,5-pentanetriol, 1,2,6-hexanetriol, 1,2,3,4-butanetetrol, sorbitol, sorbitan, a sorbitol glycerin condensate, adonitol, arabitol, xylitol, or mannitol; sugars such as xylose, arabinose, ribose, rhamnose, glucose, fructose, galactose, mannose, sorbose, cellobiose, maltose, isomaltose, trehalose, or sucrose; and the mixtures thereof.

As monobasic acids, a fatty acid having 2 to 24 carbon atoms is generally used, and the fatty acid may be linear or branched, and may also be saturated or unsaturated. Specific examples include: saturated fatty acids such as acetic acid, propionic acid, linear or branched butanoic acid, linear or branched pentanoic acid, linear or branched hexanoic acid, linear or branched heptanoic acid, linear or branched octanoic acid, linear or branched nonanoic acid, linear or branched decanoic acid, linear or branched undecanoic acid, linear or branched dodecanoic acid, linear or branched tridecanoic acid, linear or branched tetradecanoic acid, linear or branched pentadecanoic acid, linear or branched hexadecanoic acid, linear or branched heptadecanoic acid, linear or branched octadecanoic acid, linear or branched nonadecanoic acid, linear or branched icosanoic acid, linear or branched henicosanoic acid, linear or branched docosanoic acid, linear or branched tricosanoic acid, or linear or branched tetracosanoic acid; unsaturated fatty acids such as acrylic acid, linear or branched butenoic acid, linear or branched pentenoic acid, linear or branched hexenoic acid, linear or branched heptenoic acid, linear or branched octenoic acid, linear or branched nonenic acid, linear or branched decenoic acid, linear or branched undecenoic acid, linear or branched dodecenoic acid, linear or branched tridecenoic acid, linear or branched tetradecenoic acid, linear or branched pentadecenoic acid, linear or branched hexadecenoic acid, linear or branched heptadecenoic acid, linear or branched octadecenoic acid, linear or branched nonadecenoic acid, linear or branched icosenoic acid, linear or branched henicosenoic acid, linear or branched docosenoic acid, linear or branched tricosenoic acid, or linear or branched tetracosenoic acid; and the mixtures thereof.

Examples of the polybasic acid include dibasic acid having 2 to 16 carbon atoms and trimellitic acid. The dibasic acid having 2 to 16 carbon atoms may be linear or branched, and may also be saturated or unsaturated. Specific examples include ethanedioic acid, propanedioic acid, linear or branched butanedioic acid, linear or branched pentanedioic acid, linear or branched hexanedioic acid, linear or branched heptanedioic acid, linear or branched octanedioic acid, linear or branched nonanedioic acid, linear or branched decanedioic acid, linear or branched undecanedioic acid, linear or branched dodecanedioic acid, linear or branched tridecanedioic acid, linear or branched tetradecanedioic acid, linear or branched heptadecanedioic acid, linear or branched hexadecanedioic acid, linear or branched hexenedioic acid, linear or branched heptenedioic acid, linear or branched octenedioic acid, linear or branched nonenedioic acid, linear or branched decenedioic acid, linear or branched undecenedioic acid, linear or branched dodecenedioic acid, linear or branched tridecenedioic acid, linear or branched tetradecenedioic acid, linear or branched heptadecenedioic acid, linear or branched hexadecenedioic acid, and the mixtures thereof.

The combination of alcohol and acid that form an ester are arbitrarily selected, and is not particularly limited, and examples of the ester that can be used in the present invention include the following esters, and these esters may be used singly or in combinations of two or more:

(a) an ester formed from monohydric alcohol and monobasic acid,

(b) an ester formed from polyhydric alcohol and monobasic acid,

(c) an ester formed from monohydric alcohol and polybasic acid,

(d) an ester formed from polyhydric alcohol and polybasic acid,

(e) a mixed ester formed from a mixture of monohydric alcohol and polyhydric alcohol, and polybasic acid,

(f) a mixed ester formed from polyhydric alcohol, and a mixture of monobasic acid and polybasic acid, and

(g) a mixed ester formed from a mixture of monohydric alcohol and polyhydric alcohol, monobasic acid, and polybasic acid,

Among these esters, (c) the ester formed from monohydric alcohol and polybasic acid is preferable because it is excellent in abrasion resistance and oxidation stability, and a dibasic acid ester that is an ester formed from monohydric alcohol and dibasic acid is more preferable.

The content of the ester base oil is 0.5% to 70% by mass, preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, based on the total amount of the lubricating base oil. Also, it is preferably 60% by mass or less, and more preferably 55% by mass or less. If the content of the ester base oil is 0.5% by mass or more, the lubricating base oil tends to be excellent in extreme pressure properties, wear resistance, seizure resistance, and abrasion resistance. In addition, if the content of the ester base oil is 70% by mass or less, the lubricating base oil tends to be excellent in oxidation stability.

The kinematic viscosity of the ester base oil at 40° C. is not particularly limited, and it is preferably 5 mm²/s or more, more preferably 6 mm²/s or more, and even more preferably 7 mm²/s or more. Also, it is preferably 50 mm²/s or less, more preferably 30 mm²/s or less, and even more preferably 20 mm²/s or less. If the kinematic viscosity at 40° C. is 5 mm²/s or more, or 50 mm²/s or less, the lubricating base oil tends to be excellent in extreme pressure properties, wear resistance, and seizure resistance.

The viscosity index of the ester base oil is not particularly limited, and it is preferably 125 or more, more preferably 130 or more, and even more preferably 135 or more. If the viscosity index is 125 or more, the lubricating base oil tends to be excellent in low temperature fluidity.

The pour point of the ester base oil is not particularly limited, and it is preferably −30° C. or lower, more preferably −50° C. or lower, even more preferably −60° C. or lower, and particularly preferably −70° C. or lower.

The flash point of the ester base oil is not particularly limited, and it is preferably 200° C. or higher, more preferably 250° C. or higher, and even more preferably 300° C. or higher.

The lubricating base oil according to the present embodiment may comprise base oil components other than the ester base oil, as long as the content of the ester base oil is 0.5% to 70% by mass based on the total amount of the lubricating base oil. The base oil components other than the ester base oil are not particularly limited, and base oil used in ordinary lubricating oil can be used. Specific examples of the oil components other than the ester base oil that can be used herein include mineral oil base oil, synthetic base oil, and a mixture obtained by mixing two or more types of base oils selected from the aforementioned base oils at any given ratio.

Examples of the mineral oil base oil include: paraffinic and naphthenic mineral oil base oils, which are obtained by purifying a. lubricating oil fraction, which has been obtained by subjecting crude oil to atmospheric distillation and vacuum distillation, by applying purification treatments, such as solvent deasphalting, solvent extraction, hydrogenolysis, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid washing, or clay treatment, alone or appropriately in combinations of two or more; and base oils produced by subjecting normal paraffin, isoparaffin and petroleum wax to catalytic dewaxing. It is to be noted that these base oils may be used singly or in combinations of two or more at any given ratio.

From the viewpoint of reduction in viscosity and the content of sulfur, the mineral oil base oil is preferably base oil classified into Group II or Group III in Base Stock Categories of API (American Petroleum Institute), and more preferably base oil classified into Group III.

Examples of the synthetic base oil include a poly-α-olefin or a hydride thereof, an isobutene oligomer or a hydride thereof, isoparaffin, alkylbenzene, alkylnaphthalene, polyoxyalkylene glycol, dialkyl diphenyl ether, polyphenyl ether, and base oil produced by subjecting wax produced by the Fischer-Tropsch process to catalytic dewaxing.

The synthetic base oil is preferably a poly-α-olefin, or base oil produced by subjecting wax produced by the Fischer-Tropsch process to catalytic dewaxing. Specific examples of the poly-α-olefin include α-olefin oligomers or cooligomers having 2 to 32, and preferably 6 to 16 carbon atoms (e.g., a 1-octene oligomer, a 1-decene oligomer, a 1-dodecene oligomer, an ethylene-propylene cooligomer, etc.), and the hydrides thereof.

A method for producing a poly-α-olefin is not particularly limited, and an example of the production method is polymerization of an α-olefin in the presence of a polymerization catalyst comprising a complex with aluminum trichloride, boron trifluoride, or boron trifluoride and water, alcohol (e.g., ethanol, propanol or butanol), carboxylic acid, or ester (e.g., ethyl acetate or ethyl propionate), such as a Friedel-Crafts catalyst.

The kinematic viscosity of the lubricating base oil at 40° C. is 18 to 28 mm²/s, preferably 20 mm²/s or more, and more preferably 22 mm²/s or more. Also, it is preferably 27 mm²/s or less, and more preferably 26 mm²/s or less. By setting the kinematic viscosity at 40° C. to 18 mm²/s or more, an oil film is sufficiently formed, and thus, it becomes possible to obtain a lubricating oil composition, which is excellent in lubricity and has a smaller evaporation loss of the base oil under high-temperature conditions. On the other hand, by setting the kinematic viscosity at 40° C. to 28 mm²/s or less, a lubricating oil composition becomes excellent in low temperature fluidity and fluid resistance thereof is decreased, and thus, it becomes possible to obtain a lubricating oil composition having smaller rotational resistance.

The kinematic viscosity of the lubricating base oil at 100° C. is not particularly limited, and it is preferably 1 mm²/s or more, more preferably 3 mm²/s or more, and even more preferably 4 mm²/s or more. Also, it is preferably 10 mm²/s or less, more preferably 8 mm²/s or less, and even more preferably 6 mm²/s or less. By setting the kinematic viscosity at 100° C. to 1 mm²/s or more, an oil film is sufficiently formed, and thus, it becomes possible to obtain a lubricating oil composition, which is excellent in lubricity and has a smaller evaporation loss of the base oil under high-temperature conditions. On the other hand, by setting the kinematic viscosity at 100° C. to 10 mm²/s or less, it becomes possible to obtain a lubricating oil composition, which is excellent in low temperature fluidity.

The viscosity index of the lubricating base oil is not particularly limited, and it is preferably 120 or more, more preferably 125 or more, and even more preferably 130 or more. By setting the viscosity index to 120 or more, a lubricating oil composition, which exhibits good viscosity properties in a temperature range from a low temperature to a high temperature, and is excellent in oxidation stability.

[Component (B): Organic Molybdenum Compound]

The lubricating oil composition according to the present embodiment comprises, as a friction modifier, an organic molybdenum compound in an amount of 100 to 1000 mass ppm in terms of the molybdenum element based on the total amount of the lubricating oil composition. By combining the component (B) with the component (A), a metal-to-metal friction coefficient can be reduced, and fuel saving can be enhanced.

Examples of the organic molybdenum compound according to the present embodiment include: organic molybdenum compound containing sulfur, such as molybdenum dithiophosphate or molybdenum dithiocarbamate (MoDTC); complexes formed from molybdenum compounds (e.g., molybdenum oxides such as molybdenum dioxide or molybdenum trioxide, molybdic acids such as orthomolybdic acid, paramolybdic acid or polysulfurized molybdic acid, the metal salts of these molybdic acids, molybdates such as ammonium salt, molybdenum sulfides such as molybdenum disulfide, molybdenum trisulfide, molybdenum pentasulfide or molybdenum polysulfide, sulfurized molybdic acids, the metal salts or amine salts of the sulfurized molybdic acids, halogenated molybdenums such as molybdenum chloride, etc.), and sulfur-containing organic compounds (e.g., alkyl(thio)xanthate, thiaziazole, mercaptothiadiazole, thiocarbonate, tetrahydrocarbylthiuramdisulfide, bis(di(thio)hydrocarbyldithiophosphonate)disulfide, organic (poly)sulfide, and sulfide ester, etc.) or other organic compounds; and complexes formed from sulfur-containing molybdenum compounds such as the above-mentioned molybdenum sulfides or sulfurized molybdenum acids and alkenyl succinimides.

Moreover, as such an organic molybdenum compound, an organic molybdenum compound, which does not contain sulfur as a constitutional element, can be used. Specific examples of the organic molybdenum compound, which does not contain sulfur as a constitutional element, include a molybdenum-amine complex, a molybdenum-succinimide complex, the molybdenum salt of organic acid, and the molybdenum salt of alcohol, and among others, a molybdenum-amine complex, the molybdenum salt of organic acid, and the molybdenum salt of alcohol are preferable.

In the lubricating oil composition according to the present embodiment, the content of the organic molybdenum compound is 100 to 1000 mass ppm, preferably 200 mass ppm or more, and more preferably 300 mass ppm or more, in terms of the molybdenum element based on the total amount of the lubricating oil composition. Also, it is preferably 900 mass ppm or less, and more preferably 800 mass ppm or less. If the content of the organic molybdenum compound is 100 mass ppm or more, the lubricating oil composition tends to be excellent in wear resistance and abrasion resistance, and if the content is 1000 mass ppm or less, the lubricating oil composition tends to be excellent in seizure resistance. It is to be noted that the amount of the organic molybdenum compound in terms of the molybdenum element can be obtained, for example, by an ICP elemental analysis method or the like.

The lubricating oil composition according to the present embodiment may further comprise, as a viscosity modifier, a copolymer consisting of an α-olefin and an ester monomer having a polymerizable unsaturated bond, in an amount of 2% by mass or more based on the total amount of the lubricating oil composition. The weight-average molecular weight of the above described copolymer is preferably 2000 to 20000. By allowing the present lubricating oil composition to further comprise such a copolymer, the oil film retentivity and extreme pressure properties of the lubricating oil composition can be further improved.

The ester monomer having a polymerizable unsaturated bond is not particularly limited, as long as it is a compound having a polymerizable unsaturated bond and an ester bond, and the ester monomer is preferably an α,β-ethylenically unsaturated dicarboxylic acid diester, which is a diester body of unsaturated dicarboxylic acid, in which the α carbon and β carbon of at least one carboxy group form an ethylenically unsaturated bond (namely, a C═C double bond). Herein, the α,β-ethylenically unsaturated dicarboxylic acid is not limited to a compound in which an α carbon and a β carbon form an ethylenically unsaturated bond in both carboxy groups and the α,β-ethylenically unsaturated bond is present in the main chain, such as maleic acid, fumaric acid, citraconic acid or mesaconic acid, but the α,β-ethylenically unsaturated dicarboxylic acid used herein means a concept including a compound in which an α carbon and a β carbon form an ethylenically unsaturated bond in only one carboxy group, such as glutaconic acid, or a concept including a compound in which the α,β-ethylenically unsaturated bond is found in the side chain, such as itaconic acid.

The structure of the copolymer consisting of an α-olefin and an ester monomer having a polymerizable unsaturated bond is not particularly limited, as long as its weight-average molecular weight is 2000 to 20000. In addition, a method for producing the copolymer is not particularly limited, either, and a copolymer produced by a known method can be used.

The weight-average molecular weight (Mw) of the copolymer consisting of an α-olefin and an ester monomer having a polymerizable unsaturated bond is 2000 to 20000, preferably 4000 or more, and more preferably 6000 or more. Also, the weight-average molecular weight is preferably 15000 or less, and more preferably 12000 or less. By setting the weight-average molecular weight at 2000 to 20000, it becomes possible to improve oil film retentivity and extreme pressure properties.

It is to be noted that the term “weight-average molecular weight” used herein means a weight-average molecular weight relative to standard polystyrene, which is measured using, in series, two columns of GMHHR-M (7.8 mm ID×30 cm) manufactured by Tosoh Corporation in the 150-C ALC/GPC device manufactured by Waters, using tetrahydrofuran as a solvent, and also using a refractive index (RI) detector under conditions of a temperature of 23° C., a flow rate of 1 mL/min., a sample concentration of 1% by mass, and a sample injection rate of 75 μL.

In the lubricating oil composition according to the present embodiment, the content of the copolymer is preferably 2% by mass or more, more preferably 2.5% by mass or more, and even more preferably 3.5% by mass or more, based on the total amount of the lubricating oil composition. By setting the content of the copolymer to 2% by mass or more, the present lubricating oil composition tends to be excellent in extreme pressure properties and wear resistance. On the other hand, the upper limit of the content is not particularly limited, and it is preferably 25% by mass or less, more preferably 24% by mass or less, and even more preferably 22% by mass or less. By setting the content of a component (D) to 25% by mass or less, the lubricating oil composition tends to exhibit sufficient extreme pressure properties, wear resistance, seizure resistance, abrasion resistance and oxidation stability.

The lubricating oil composition according to the present embodiment may further comprise a boron-containing dispersant in an amount of 100 to 500 mass ppm in terms of the boron element based on the total amount of the lubricating oil composition. Thereby, the oil film retentivity and extreme pressure properties of the present lubricating oil composition can be further improved.

The boron-containing dispersant is any given ashless dispersant that has been borated. Examples of the ashless dispersant include a nitrogen-containing compound having at least one linear or branched alkyl group or alkenyl group having 40 to 400 carbon atoms in the molecule thereof, or a derivative thereof, and a modified product of alkenyl succinimide. One or more types arbitrarily selected from these products may be blended into the lubricating oil composition.

It is to be noted that the succinimide includes, what is called, a mono-type succinimide represented by the following formula (3), in which succinic anhydride is added to one end of polyamine, and what is called, a bis-type succinimide represented by the following formula (4), in which succinic anhydride is added to both ends of polyamine.

In the above formula (3), R⁹ represents an alkyl group or alkenyl group having 40 to 400 carbon atoms, and preferably an alkyl group or alkenyl group having 60 to 350 carbon atoms; and p represents an integer of 1 to 5, and preferably of 2 to 4.

In the above formula (4), R¹⁰ and R¹¹ may be the same or different, and each represent an alkyl group or alkenyl group having 40 to 400 carbon atoms, preferably an alkyl group or alkenyl group having 60 to 350 carbon atoms, and preferably, each represent a polybutenyl group; and q represents an integer of 0 to 4, and preferably of 1 to 3.

The lubricating oil composition according to the present embodiment may comprise either one of the mono-type and bis-type succinimides, or may also comprise both of them.

A method for producing a succinimide is not particularly limited, and the succinimide can be obtained, for example, by allowing alkyl succinic acid or alkenyl succinic acid, which has been obtained by reacting a compound having an alkyl group or alkenyl group having 40 to 400 carbon atoms with maleic anhydride at a temperature of 100° C. to 200° C., to react with polyamine. Specific examples of the polyamine include diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine.

In the lubricating oil composition according to the present embodiment, the content of the boron-containing dispersant is preferably 100 to 500 mass ppm, more preferably 150 mass ppm or more, and even more preferably 200 mass ppm or more, in terms of the boron element based on the total amount of the lubricating oil composition. Also, it is more preferably 450 mass ppm or less, and even more preferably 400 mass ppm or less. If the content of the boron-containing dispersant in the lubricating oil composition is 100 mass ppm or more, the present lubricating oil composition tends to be excellent in extreme pressure properties, wear resistance, seizure resistance, and abrasion resistance. Also, if the content of the boron-containing dispersant in the lubricating oil composition is 500 mass ppm or less, the present lubricating oil composition tends to be excellent in wear resistance. It is to be noted that the amount of the boron-containing dispersant in terms of the boron element can be obtained, for example, by an ICP elemental analysis method or the like.

In order to improve performance, the lubricating oil composition according to the present embodiment may further comprise any given additives, which are commonly used in lubricating oil, depending on purpose. Examples of such additives include viscosity modifiers other than the above described copolymer, metallic detergents, ashless dispersants other than boron-containing dispersants, wear inhibitors (or extreme pressure agents), antioxidants, corrosion inhibitors, rust inhibitors, demulsifiers, metal deactivators, defoaming agents, and friction modifiers other than the component (B).

Specific examples of the viscosity modifier other than the above described copolymer include viscosity modifiers containing a non-dispersed or dispersed ester group, such as a non-dispersed or dispersed poly(meth)acrylate viscosity modifier, a non-dispersed or dispersed olefin-(meth)acrylate copolymer viscosity modifier, a styrene-maleic anhydride ester copolymer viscosity modifier, and the mixtures thereof; and among others, a non-dispersed or dispersed poly(meth)acrylate viscosity modifier is preferable. In particular, a non-dispersed or dispersed polymethacrylate viscosity modifier is preferable.

Other examples of the viscosity modifier other than the above described copolymer include a non-dispersed or dispersed ethylene-α-olefin copolymer or a hydride thereof, polyisobutylene or a hydride thereof, a styrene-diene hydrogenated copolymer, and polyalkylstyrene.

Examples of the metallic detergent include a sulfonate detergent, a salicylate detergent and a phenate detergent, and any one of normal salt, basic normal salt, and perbasic salt with alkaline metal or alkaline earth metal can be blended. Upon the use, one or two or more types, which are arbitrarily selected from these substances, can be blended into the lubricating oil composition.

As ashless dispersants other than the boron-containing dispersant, any given non-boron ashless dispersants, which are used in lubricating oil, can be used, and examples of such an ashless dispersant include a mono- or bis-succinimide having at least one linear or branched alkyl group or alkenyl group having 40 to 400 carbon atoms in the molecule thereof, a benzylamine having at least one alkyl group or alkenyl group having 40 to 400 carbon atoms in the molecule thereof, a polyamine having at least one alkyl group or alkenyl group having 40 to 400 carbon atoms in the molecule thereof, and the modified products thereof, which are prepared by using carboxylic acid, phosphoric acid or the like. Upon the use, one or two or more types, which are arbitrarily selected from these substances, can be blended into lubricating oil composition.

As wear inhibitors (or extreme pressure agents), any given wear inhibitors and/or extreme pressure agents, which are used in lubricating oil, can be used. For example, sulfur-based, phosphorus-based, and sulfur-phosphorus-based extreme pressure agents and the like can be used, and specific examples of such an extreme pressure agent include zinc dialkyldithiophosphate (ZnDTP), phosphite esters, thiophosphite esters, dithiophosphite esters, trithiophosphite esters, phosphoric acid esters, thiophosphoric acid esters, dithiophosphoric acid esters, trithiophosphoric acid esters, the amine salts thereof, the metal salts thereof, the derivatives thereof, dithiocarbamate, zinc dithiocarbamate, MoDTC, disulfides, polysulfides, sulfurized olefins, and sulfurized oils and fats. Among these substances, addition of sulfur-based extreme pressure agents is preferable, and sulfurized oils and fats are particularly preferable.

Examples of the antioxidant include ashless antioxidants such as phenolic ashless antioxidants or amine-based ashless antioxidants, and metallic antioxidants such as copper-based or molybdenum-based antioxidants. Specific examples of the phenolic ashless antioxidant include 4,4′-methylenebis(2,6-di-tert-butylphenol) and 4,4′-bis(2,6-di-tert-butylphenol; and examples of the amine-based ashless antioxidant include phenyl-α-naphthylamine, alkylphenyl-α-naphthylamine, and dialkyldiphenylamine.

Examples of the corrosion inhibitor include benzotriazole-based, tolyltriazole-based, thiadiazole-based, and imidazole-based compounds.

Examples of the rust inhibitor include petroleum sulfonate, alkylbenzene sulfonate, dinonylnaphthalene sulfonate, alkenyl succinic acid ester, and polyhydric alcohol ester.

Examples of the demulsifier include polyalkylene glycol-based nonionic surfactants such as polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, or polyoxyethylene alkylnaphthyl ether.

Examples of the metal deactivator include imidazoline, a pyrimidine derivative, alkylthiadiazole, mercaptobenzothiazole, benzotriazole or a derivative thereof, 1,3,4-thiadiazole polysulfide, 1,3,4-thiadiazolyl-2,5-bisdialkyldithiocarbamate, 2-(alkylthio)benzoimidazole, and β-(o-carboxybenzylthio)propionitrile.

Examples of the defoaming agent include silicone oil, an alkenyl succinic acid derivative, an ester of polyhydroxy aliphatic alcohol and a long chain fatty acid, and an ester of methyl salicylate and o-hydroxybenzyl alcohol, of which kinematic viscosity at 25° C. is 1000 to 100000 mm²/s, respectively.

Examples of the friction modifier other than the component (B) include ashless friction modifiers, and any given compounds, which are generally used as ashless friction modifiers for lubricating oil, can be used, and examples of such an ashless friction modifier include amine-based, imide-based, fatty acid ester-based, fatty acid amide-based, fatty acid-based, aliphatic alcohol-based, and aliphatic ether-based ashless friction modifiers, each of which has at least one hydrocarbon group having 6 to 30 carbon atoms, preferably at least one alkyl group or alkenyl group, and particularly preferably at least one linear alkyl group or linear alkenyl group having 6 to 30 carbon atoms, in the molecule thereof.

When these additives are contained in the lubricating oil composition according to the present embodiment, the content of each additive is preferably 0.01% to 20% by mass based on the total amount of the lubricating oil composition.

The kinematic viscosity of the lubricating oil composition according to the present embodiment at 40° C. is 50 mm²/s or less, preferably 48 mm²/s or less, and more preferably 45 mm²/s or less. By setting the kinematic viscosity at 40° C. to 50 mm²/s or less, necessary low temperature fluidity and sufficient fuel savings tend to be obtained. Moreover, the lower limit of the kinematic viscosity of the lubricating oil composition according to the present embodiment at 40° C. is not particularly limited, and it is preferably 20 mm²/s or more, more preferably 30 mm²/s or more, and even more preferably 35 mm²/s or more. By setting the kinematic viscosity at 40° C. to 20 mm²/s or more, the present lubricating oil composition tends to be excellent in oil film retentivity and evaporativity at lubrication sites.

Since the lubricating oil composition according to the present embodiment has extreme pressure properties and wear resistance, which are sufficient for achieving fuel saving, and is further capable of reducing a metal-to-metal friction coefficient, it can be preferably used as a gear oil for an automotive manual transmission, an automatic transmission or a continuously variable transmission, or for an industrial gear system, and in particular, as a hypoid gear oil for the driving systems of automobiles and railway vehicles.

EXAMPLES

Hereinafter, the present invention will be more specifically described in the following examples. However, these examples are not intended to limit the scope of the present invention.

Examples 1 to 18 and Comparative Examples 1 to 4

As shown in Table 1 and Table 2, the lubricating oil compositions of Examples 1 to 18 and Comparative Examples 1 to 4 were prepared, respectively. With regard to the obtained lubricating oil compositions, their extreme pressure properties, wear resistance, seizure resistance, abrasion resistance, and oxidation stability were measured, and the results are shown in Table 1 and Table 2.

Details of individual components shown in Table 1 and Table 2 are as follows.

Base oil A-1: poly-α-olefin [Group IV, 40° C. kinematic viscosity: 19 mm²/s, 100° C. kinematic viscosity: 4.1 mm²/s, viscosity index: 126, pour point: −66° C., flash point: 220° C.]

Base oil A-2: poly-α-olefin [Group IV, 40° C. kinematic viscosity: 30.3 mm²/s, 100° C. kinematic viscosity: 5.9 mm²/s, viscosity index: 142, pour point: <−54° C., flash point: 246° C.]

Base oil A-3: poly-α-olefin [Group IV, 40° C. kinematic viscosity: 48 mm²/s, 100° C. kinematic viscosity: 8.0 mm²/s, viscosity index: 139, pour point: −48° C., flash point: 260° C.]

Base oil A-4: poly-α-olefin [Group IV, 40° C. kinematic viscosity: 396 mm²/s, 100° C. kinematic viscosity: 39 mm²/s, viscosity index: 147, pour point: −36° C., flash point: 281° C.]

Base oil A-5: hydrorefined mineral oil [Group III, 40° C. kinematic viscosity: 33.97 mm²/s, 100° C. kinematic viscosity: 6.208 mm²/s, viscosity index: 133, sulfur content: less than 10 mass ppm, % C_(P): 80.6, % C_(N): 19.4, % C_(A): 0]

Base oil B-1: dibasic acid ester [Group V, azelaic acid+2-ethylhexanol, 40° C. kinematic viscosity: 10.3 mm²/s, 100° C. kinematic viscosity: 2.9 mm²/s, viscosity index: 138, pour point: −72° C., flash point: 220° C.]

Organic molybdenum compound F-1: molybdenum dithiocarbamate (MoDTC) [amount in terms of the molybdenum element: 10% by mass]

Boron-containing dispersant G-1: borated succinimide [amount in terms of the boron element: 2.0% by mass, amount in terms of nitrogen element: 2.3% by mass, weight-average molecular weight: 1000]

Non-boron dispersant H-1: succinimide [amount in terms of nitrogen element: 2.3 by mass, weight-average molecular weight: 1000]

Performance additive C-1: an additive package comprising a phosphorus wear inhibitor, a sulfur extreme pressure agent, a metal deactivator, a friction modifier, a defoaming agent, etc., [amount in terms of phosphorus element: 1.40% by mass, amount in terms of sulfur element: 22.9% by mass]

Viscosity modifier J-1: a copolymer of an α-olefin and an α,β-ethylenically unsaturated dicarboxylic acid diester [weight-average molecular weight: 10000]

Viscosity modifier J-2: a copolymer of an α-olefin and an α,β-ethylenically unsaturated dicarboxylic acid diester [weight-average molecular weight: 7000]

Viscosity modifier J-3: an oligomer of ethylene and α-olefin [number-average molecular weight: 3700]

The amount of the organic molybdenum compound in terms of the molybdenum element, the amount of the boron-containing dispersant in terms of the boron element, the amount of the performance additive in terms of phosphorus element, and the amount of the performance additive in terms of sulfur element were obtained by an ICP elemental analysis method.

(1) Extreme Pressure Property Test

In accordance with ASTM D 2596, using a high speed four-ball tester, the maximum non-seizure load (LNSL) at 1800 rotation of each lubricating oil composition was measured. In the present test, it means that the larger the maximum non-seizure load, the better the extreme pressure properties that can be achieved.

(2) Wear Resistance Test

A four-ball test (ASTM D 4172) was carried out under the below-mentioned conditions, and a wear scar diameter (mm) was then measured, so that wear resistance was evaluated. In the present test, it means that the smaller the wear scar diameter, the better the wear resistance that can be achieved.

Load: 800 N

Number of rotations: 1800 rpm

Temperature: 80° C.

Testing time: 30 minutes

(3) Seizure Resistance Test

Using a Falex Tester described in ASTM D 3233, a seizure load was measured, and seizure resistance was evaluated. This seizure resistance indicates an extreme pressure property between steels. Conditions for the test are described below. In the present test, it means that the larger the seizure load, the better the seizure resistance that can be achieved.

Temperature: 110° C.

Number of rotations: 290 rpm

(4) Abrasion Resistance Test

Using a Block on Ring Tester (LFW-1) described in ASTM D 2174, a friction coefficient was measured under the below-mentioned test conditions. Moreover, in the present test, such a friction coefficient was obtained from both a new oil of the lubricating oil composition and a degraded oil thereof, which had been obtained by subjecting the lubricating oil to the after-mentioned oxidation stability test at 135° C. for 48 hours. In the present test, it means that the smaller the friction coefficient, the better the abrasion resistance that can be achieved.

Ring: Falex S-10 Test Ring (SAE4620 Steel)

Block: Falex H-60 Test Block (SAE01 Steel)

Oil temperature: 90° C.

Load: 222-3113 N

Slip velocity: 0.5 m/s

(5) Oxidation Stability Test

In accordance with JIS K 2514 4. (a method for testing the oxidation stability of lubricating oil for internal combustion engines), the test was carried out under the below-mentioned conditions, and an acid value increase was measured. In the present test, it means that the smaller the acid value increase, the better the oxidation stability that can be achieved.

Temperature: 135° C.

Testing time: 96 hours

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Base oil A-1 % by mass — 40 32 — — — — — — Base oil A-2 % by mass 80 59 63 — — — 80 80 80 Base oil A-3 % by mass — — — 40 — — — — — Base oil A-4 % by mass — — — 10 30 — — — — Base oil A-5 % by mass — — — — — 80 — — — Base oil B-1 % by mass 20 1 5 50 70 20 20 20 20 Base oil (mixture) kinematic viscosity  (40° C.) mm²/s 23.72 24.66 24.41 23.45 22.65 25.83 23.72 23.72 23.72 (100° C.) mm²/s 5.03 5.03 5.02 5.05 5.05 5.22 5.03 5.03 5.03 Base oil (mixture) viscosity index 144 134 136 149 158 138 144 144 144 Additive composition (based on total amount of composition) Organic molybdenum compound F-1 % by mass 0.5 (500) 0.5 (500) 0.5 (500) 0.5 (500) 0.5 (500) 0.5 (500) 0.1 (100) 0.2 (200) 0.7 (700) (amount in terms of the molybdenum element) mass ppm Boron-containing dispersant G-1 % by mass 1.0 (200) 1.0 (200) 1.0 (200) 1.0 (200) 1.0 (200) 1.0 (200) 1.0 (200) 1.0 (200) 1.0 (200) (amount in terms of boron element) mass ppm Non-boron dispersant II-1 % by mass — — — — — — — — — Performance additive C-1 % by mass 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 Viscosity modifier J-1 % by mass 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 Viscosity modifier J-2 % by mass — — — — — — — — — Viscosity modifier J-3 % by mass 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 Kinematic viscosity  (40° C.) mm²/s 40.0 41.1 40.9 39.7 38.9 43.1 40.0 40.0 40.0 High speed four-ball test 1800 rpm LNSL N 981 981 981 1236 1236 981 981 981 981 Wear scar diameter mm 0.93 0,96 0.95 0.85 0.80 0.97 0.97 0.95 0.92 Falex seizure test N 4893 4671 4893 6895 7918 4715 5382 5249 4671 IFW-1 test (friction coefficient) (New oil) 90° C. 0.089 0.098 0.094 0.087 0.088 0.091 0.091 0.090 0.090 (Degraded oil) 90° C. 0.030 0.045 0.040 0.031 0.035 0.033 0.046 0.041 0.031 Oxidation stability (ISOT) (Acid value increase) mgKOH/g 2.44 2.04 2.11 2.72 2.89 2.81 2.41 2.41 2.51 Example Example Example Example Example 10 11 Example 12 13 Example 14 15 Example 16 17 Example 18 Base oil A-1 % by mass — — — — — — — — — Base oil A-2 % by mass 80 80 80 80 80 80 80 80 80 Base oil A-3 % by mass — — — — — — — — — Base oil A-4 % by mass — — — — — — — — — Base oil A-5 % by mass — — — — — — — — — Base oil B-1 % by mass 20 20 20 20 20 20 20 20 20 Base oil (mixture) kinematic viscosity  (40° C.) mm²/s 23.72 23.72 23.72 23.72 23.72 23.72 23.72 23.72 23.72 (100° C.) mm²/s 5.03 5.03 5.03 5.03 5.03 5.03 5.03 5.03 5.03 Base oil (mixture) viscosity index 144 144 144 144 144 144 144 144 144 Additive composition (based on total amount of composition) Organic molybdenum compound F-1 % by mass  1.0 (1000) 0.5 (500) 0.5 (500) 0.5 (500) 0.5 (500) 0.5 (500) 0.5 (500) 0.5 (500) 0.5 (500) (amount in terms of the molybdenum element) mass ppm Boron-containing dispersant G-1 % by mass 1.0 (200) 0.5 (100) 2.5 (500) 1.0 (200) 1.0 (200) 1.0 (200) 1.0 (200) 1.0 (200) 1.0 (200) (amount in terms of boron element) mass ppm Non-boron dispersant H-1 % by mass — — — — — — — — — Performance additive C-1 % by mass 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 Viscosity modifier J-1 % by mass 3.5 3.5 3.5 2.5 7.0 10.0 15.0 5.0 — Viscosity modifier J-2 % by mass — — — — — — — 17.0 22.0 Viscosity modifier J-3 % by mass 4.7 4.7 4.7 5.3 3.0 1.0 — — — Kinematic viscosity  (40° C.) mm²/s 40.0 39.9 41.0 39.8 39.4 38.7 39.9 40.8 40.8 High speed four-ball test 1800 rpm LNSL N 1236 981 1236 981 981 981 981 981 981 Wear scar diameter mm 0.94 0.97 0.85 0.90 0.95 0.96 0.94 0.92 0.91 Falex seizure test N 4359 4537 6672 4893 4493 4404 4359 5160 5382 IFW-1 test (friction coefficient) (New oil) 90° C. 0.091 0.087 0.093 0.090 0.096 0.095 0.093 0.094 0.095 (Degaded oil) 90° C. 0.036 0.030 0.043 0.031 0.038 0.042 0.033 0.035 0.031 Oxidation stability (ISOT) (Acid value increase) mgKOH/g 2.64 2.45 2.45 2.42 2.43 2.45 2.48 2.47 2.45

TABLE 2 Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 Base oil A-1 % by mass 42 — — — Base oil A-2 % by mass 58 — 80 80 Base oil A-3 % by mass — — — — Base oil A-4 % by mass — 20 — — Base oil A-5 % by mass — — — — Base oil B-1 % by mass — 80 20 20 Base oil (mixture) kinematic viscosity  (40° C.) mm²/s 24.72 17.04 23.72 23.72 (100° C.) mm²/s 5.03 4.13 5.03 5.03 Base oil (mixture) viscosity index 134 151 144 144 Additive composition (based on total amount of composition) Organic molybdenum compound F-1 % by mass 0.5 (500) 0.5 (500) — 1.2 (1200) (Amount in terms of the molybdenum element) mass ppm — Boron-containing dispersant G-1 % by mass 1.0 (200) 1.0 (200) 1.0 (200) 1.0 (200)  (Amount in terms of boron element) mass ppm Non-boron dispersant H-1 % by mass — — — — Performance additive C-1 % by mass 9.0 9.0 9.0 9.0 Viscosity modifier J-1 % by mass 3.5 3.5 3.5 3.5 Viscosity modifier J-2 % by mass — — — — Viscosity modifier J-3 % by mass 4.7 4.7 4.7 4.7 Kinematic viscosity  (40° C.) mm²/s 41.2 33.3 40.0 40.1 High speed four-ball test 1800 rpm LNSL N 785 1236 981 1236 Wear scar diameter mm 1.23 0.95 1.11 0.96 Falex seizure test N 3870 8363 5338 4003 IFW-1 test (friction coefficient) (New oil) 90° C. 0.098 0.088 0.090 0.094 (Degraded oil) 90° C. 0.095 0.036 0.091 0.048 Oxidation stability (ISOT) (Acid value mgKOH/g 2.1 3.11 2.23 2.83 increase)

As is apparent from Table 1 and Table 2, it was found that the lubricating oil compositions of Examples 1 to 18 were better in extreme pressure properties, wear resistance, seizure resistance, abrasion resistance and oxidation stability, and had a smaller metal-to-metal friction coefficient, than the lubricating oil compositions of Comparative Examples 1 to 4. 

1. A lubricating oil composition comprising: a lubricating base oil comprising 0.5% to 70% by mass of an ester base oil based on the total amount of the lubricating base oil, and having a kinematic viscosity at 40° C. of 18 to 28 mm²/s; and an organic molybdenum compound in an amount of 100 to 1000 mass ppm in terms of molybdenum element based on the total amount of the lubricating oil composition, wherein the lubricating oil composition has a kinematic viscosity at 40° C. 10 of 50 mm²/s or less.
 2. The lubricating oil composition according to claim 1, further comprising 2% by mass or more of a copolymer consisting of an α-olefin and an ester monomer having a polymerizable unsaturated bond based on the total amount of the lubricating oil composition, wherein the weight-average molecular weight of the copolymer is 2000 to
 20000. 3. The lubricating oil composition according to claim 1, further comprising a boron-containing dispersant in an amount of 100 to 500 mass ppm in terms of boron element based on the total amount of the lubricating oil composition.
 4. The lubricating oil composition according to claim 1, wherein the lubricating oil composition is used for a hypoid gear.
 5. The lubricating oil composition according to claim 1, wherein the lubricating base oil comprises a synthetic base oil comprising a poly-α-olefin. 